Offer an Undo capability.

In reality this is a property of the application for which speech is an interface. Some states are reversible, others may not be or may require complex book-keeping.

1. Undo may not be necessary for stateless applications.

   The cost of a single transaction governs the usability of a speech interface in this case. A high cost for a transaction (whether primary or undo) decreases usability.

2. Undo is sometimes referred to as Disconfirmation of an input.